Reading a String:

Reading a string is accomplished by Function 0AH INT 21H. DOS function 0AH
will accept a string of text entered at the keyboard and copy that string
into a memory buffer.
DOS 0AH is invoked with DS:DX pointing to an input
buffer, whose size should be at least three bytes longer than the
largest input string anticipated.
Before invoking DOS function 0AH, you must set the
first byte of the buffer with the number of character spaces in the buffer.
After returning from DOS function 0AH, the second byte of the buffer will
contain a value giving the number of characters actually read form the
keyboard (see table).